The present invention relates to an air-fuel ratio control apparatus in which the air-fuel ratio is controlled by a feedback system as well as by an open-loop system.
Conventionally, in order to reduce harmful components in the exhaust gas of an engine, a method has been utilized in which such components in the exhaust gas are detected to effect feedback control to maintain a proper air-fuel ratio.
In this method, for example, as shown in FIG. 1, the existence of oxygen in exhaust components is detected by an exhaust component detector 3 provided in an exhaust pipe 100 of an engine 1, in order to control exhaust components so as to be maintained below a desired value.
Particularly, control is performed such that the basic fuel feed amount, which is determined in accordance with an output signal of an air quantity sensor 2 for detecting the intake air amount of the engine 1, is corrected on the basis of information obtained from the exhaust component detector 3, and a fuel control value 6 is controlled by means of a change-over device 11.
Since the output of the conventionally commonly used exhaust component detector 3 is reversed at the point of a stoichiometric air-fuel ratio of the exhaust components as shown in FIG. 2, integration is effected in a feedback control circuit 7 so that the fuel amount fed to the engine 1 from the fuel control valve 6 alternates between rich and lean, centering around an average value Qm.
Thus, the exhaust gas can be purified, since the air-fuel ratio is maintained in a range in which an exhaust emission control device can perform the purifying operation with optimum efficiency by feedback control of the air-fuel ratio.
On the other hand, in the control of a motor vehicle engine, there is also a range in which the output of the engine must take precedence over exhaust gas purification. For example, as shown in FIG. 3, it is common that, in accordance with the relation between the engine R.P.M. and the engine output, feedback control is normally used in zone FB, while in a high load region, i.e. a power enrich zone PE, open-loop control is performed.
For example, the load condition of the engine is determined by a load determination circuit 10 on the basis of signals obtained from the air quantity sensor 2 and an ignition device 5 which is in accordance with the engine R.P.M., and the change-over device 11 is switched to a fixed contact a in a low load state, and to the other fixed contact b in a high load state, in response to the output of the load determination circuit 10.
When the change-over device 11 is switched to the fixed contact a side, the air-fuel ratio is feedback controlled as described above, while when it is switched to the fixed contact b side, the fuel amount is open-loop controlled such that air-fuel ratio is controlled to a predetermined value by a control circuit 9.
In open-loop control, an error may be caused due to errors in the air quantity sensor 2, the fuel control valve 6, etc., and therefore, in order to correct this error, a feedback correction coefficient with respect to the basic fuel control amount during feedback control in the FB range is read and stored in a memory circuit 8 so as to perform open-loop control.
By using such means as described above, the initial error in and the change with time of fuel control valve 6 and/or the air quantity sensor 2 may be corrected to a certain extent. In the PE zone as shown in FIG. 3, however, since the correction is made on the basis of the results of feedback control in the FB zone, the correction is not always proper. This is because the operational range of each of the air quantity sensor 2 and the fuel control valve 6 varies between the FB and PE zones, and therefore in many cases the rate or tendency of error also varies so that the error cannot be completely corrected in the PE zone, making it impossible to prevent variations from occurring in the air-fuel ratio.